1. Field of the Invention
The present invention relates to an element which emits light by converting electric energy into light, and more particularly to an organic electroluminescent element which can be suitably used in the fields of display elements, back lights of liquid crystals, illumination light sources, exposing apparatus for electrophotography, labels, sign boards and the like.
2. Description of the Related Art
Since an electroluminescent element is a spontaneously emitting wholly solid element, has high visibility, and withstands impacts, wide application thereof is expected. Currently, an electroluminescent element using an inorganic fluorescent substance is popular and is used widely. Since such an electroluminescent element requires alternating current voltage in a range of 200 V or greater and 50 to 1000 Hz for driving, it has a problem in that a running cost thereof is high and luminance is insufficient. On the other hand, study of an electroluminescent element using an organic compound first started using a single crystal of anthracene or the like, but a film thickness of the element is as great as 1 mm, and the element requires a driving voltage of 100 V or greater. For that reason, as reported in Thin Solid Films, 94, 171(1982), thinning by a deposition method has been tried.
Light emission by these electroluminescent elements is a phenomenon in which electrons are injected from one electrode and holes are injected from another electrode, whereby, a light emitting material in an electroluminescent element is excited to a high energy level, and an excited light emitting substance releases, as light, extra energy produced when returned to a ground state. However, in these electroluminescent elements, a driving voltage is still as high as 30 V, densities of electron carriers and hole carriers present in a film are low, and a probability of production of photons by recombination of carriers is low. Therefore, sufficient luminance cannot be obtained, and these electroluminescent elements have not been put to practical use.
However, in 1997, Tang et al. reported in Appl. Phys. Lett., 51, 913(1987) and Japanese Patent Application Laid-Open (JP-A) No. 59-194393 that a high luminance of 1000 cd/m2 or larger can be obtained under a low driving voltage of around 10 V by an organic electroluminescent element. This organic electroluminescent element is a function separated-type organic electroluminescent element in which a hole transporting organic low-molecular compound, and a fluorescent organic low-molecular compound having electron transporting ability are successively deposited in order, as an extremely thin film, on a transparent substrate by a vacuum deposition method. Since this study by Tang et al., organic luminescence elements have been actively studied and developed.
These multilayer electroluminescent elements have a structure in which an organic light emitting substance and a charge transporting organic compound (charge transporting material) are deposited on an electrode, and the light emission by these elements is caused by recombination of holes and electrons, which are generated in both materials and move in the charge transporting material.
In this case, as the organic light emitting substance, organic dyes emitting fluorescene such as a 8-quinlinol aluminium complex and a coumarin compound are used. In addition, examples of the charge transporting material include diamino compounds such as N,N-di(m-tolyl) N,N′-diphenylbenzidine and 1,1-bis[N,N-di(p-tolyl)aminophenyl]cyclohexane, a 4-(N,N-diphenyl)aminobenzaldehyde-N,N-diphenylhydrazone compound and the like.
Organic electroluminescent elements using these organic compounds have high light emitting properties, but they have problems with respect to heat stability upon light emission and the storage stability. A thickness of a layer formed of an organic compound in an electroluminescent element is very thin, being as thin as several tens to several hundred of nanometers. Therefore, a voltage applied per unit thickness of a layer composed of this organic compound becomes very high, and the element is driven at a high current density of a few mA/cm2. Therefore, a large amount of Joule heat is generated from this layer. For this reason, a hole transporting low-molecular compound and a fluorescent organic low-molecular compound which are formed into an amorphous film are gradually crystallized as a temperature thereof is elevated and, finally, those compounds are melted, thereby causing a reduction in luminance and dielectric breakdown in many cases. Therefore, there is a problem in that a lifetime of the organic electroluminescent element is reduced. It is considered that the low heat stability of this organic electroluminescent element is derived from a low glass transition temperature of a material used in an organic electroluminescent element. That is, low-molecular compounds used as such a material have a low melting point and have high symmetry.
Thus, in order to solve the problem regarding heat stability, an organic electroluminescent element using N,N-di(1-naphthyl)N,N′-diphenylbenzidine with an α-naphthyl group introduced therein, as a material which has an improved glass transition temperature and can obtain a stable amorphous state, has been reported in Technical Study Report of the Electronic Information Communication Society, OME95-54(1995) and, for the same purpose, an organic electroluminescent element using a star burst amine has been reported in Proceedings of the 40th Applied Physics Related Associated Seminar 30a-SZK-14(1993). However, when these materials are used alone, since there is an energy barrier derived from ionization potential of a hole transporting material, hole injectability from an anode or hole injectability into a light emitting layer can not be satisfied. Further, in a 2-layered type element in which a hole transporting layer and a light emitting layer are deposited, a mutual diffusion phenomenon is caused. In addition, it is required that a material used in an organic electroluminescent element has heat stability with respect to heat treatment during a step of preparing an organic electroluminescent element, such as deposition, baking, annealing, wiring, sealing and the like, and with respect to change over time due to long term use of the organic electroluminescent element. For this reason, it is desired that a glass transition temperature of a material used in an organic electroluminescent element is even higher.
On the other hand, an electroluminescent element using a high-molecular material in place of a low-molecular compound has also been studied and developed. For example, in Nature, 357,577(1992) and JP-A No. 10-92576, there has been proposed an element using an electrically conductive polymer such as poly(p-phenylenevinylene) or the like. In addition, in Proceedings of the 42nd Polymer Discussion Meeting 20J21(1993), there has been proposed an element using a polymer in which triphenylamine is introduced on a side chain of polyphosphazene. Further, in Proceedings of the 38th Applied Physics Related Associated Seminar 31p-GI-2(1991), there has been proposed an element using a material in which an electron transporting material and a fluorescent dye are mixed into hole transporting polyvinylcarbazole.
These high-molecular materials have a comparatively higher glass transition points than that of a low-molecular compound. However, since poly(p-phenylenevinylene) is subjected to heat treatment after spin coating with a soluble precursor, defects are easily generated in a conjugated polymer on a main chain and light emitting properties are remarkably reduced. In addition, phosphazene has a high ionization potential, and thus, charge injecting properties are reduced. Further, polyvinylcarbazole has a high glass transition point, but has a problem due to traps. In addition, when a low-molecular compound is mixed into polyvinylcarbazole, the low-molecular compound acts as a plasticizer. For this reason, an organic electroluminescent element using these high-molecular materials has not yet caught up with the multilayer-type electroluminescent element using an organic low-molecular compound, in luminance, light emitting efficiency and the like.
In addition, a method of preparing an organic electroluminescent element is desirably a coating method in view of simplification of preparation, processibility, enlargement, cost and the like and, additionally, Proceedings of the 50th Applied Physics Society Seminar, 29p-ZP-5(1989) and Proceeding of the 51th Applied Physics Society Seminar, 28a-PB-7(1990) report that an organic electroluminescent element can also be prepared by a casting method. However, since a charge transporting material has deteriorated solubility in or compatibility with a solvent or a resin, the charge transporting material is easily crystallized and, thus, the above preparation methods lead to defects in preparation and properties of an organic electroluminescent element.